1. Field of the Invention
This invention relates to a liquid crystal display device and more particularly to an active liquid crystal display device capable of enhancing an aperture ratio and an image quality of the liquid crystal display device and a method of fabricating thereof.
2. Description of the Related Art
In response to a demand for personalized, space-saving displays which serve as the interface between humans and computers, various types of flat screen or flat panel displays, such as a liquid crystal display (LCD), a plasma display panel (PDP), and an electroluminescent (EL) display, etc., have been developed to replace conventional display devices, particularly a cathode-ray tube (CRT) which is relatively large and obtrusive.
Liquid crystal displays can have a simple matrix form or an active matrix form, using an electro-optic property of the liquid crystal whose molecular arrangement is varied according to an electric field. In particular, the LCD in the active matrix form utilizes a combination of liquid crystal technology and semiconductor technology, and is recognized as being superior to CRT displays.
The active matrix LCDs utilize an active device having a non-linear characteristic in each of a plurality of pixels arranged in a matrix configuration, using the switching characteristic of the device thereby to control each pixel.
A thin film transistor(hereinafter referred as a "TFT") having three electrode terminals is ordinarily used as the active device, or a thin film diode (TFD), for example, a metal insulator metal type (NIM) having two terminals, is used.
To obtain uniformity of an image displayed on the active matrix LCD, it is necessary that the voltage of a signal applied through a display signal line in a writing operation is held constant for a certain time until a second signal is received. In order to obtain this effect, a storage capacitor is formed at each pixel. Also, in order to improve the image quality of the display, a storage capacitor is formed in parallel with a liquid crystal cell.
Here, the storage capacitor is separately formed in an independent wiring form from a conventional scanning signal line. A capacitor electrode for forming the storage capacitor in an independent wiring form is formed simultaneously with the scanning signal line.
FIG. 1 shows a pixel layout of a conventional active matrix liquid crystal display on which storage capacitors in the independent wiring form is formed.
As shown in FIG. 1, scanning signal lines 2 and display signal lines 5 are arranged in a matrix form on a lower glass substrate 1. Pixels are formed at regions bounded by these two types of lines and at each pixel is arranged a pixel electrode 4 which is of a transparent material such as an indium tin oxide.
In addition, semiconductor layers 3 which serve as the channels of TFTs are formed on scanning signal lines 2. Each of Semiconductor layer 3 is connected to display signal line 5 via a drain electrode 6 and also connected to a pixel electrode 4 via a source electrode 7. Accordingly, driving of each pixel which is arranged in a matrix form is independently controlled by TFT which serves as a switching device.
Also, an independently wired capacitor electrode 10 separated from each scanning signal line 2 with a selected distance and parallel with it, is formed at each pixel. The capacitor electrode 10 is formed of opaque conductive metal such as aluminum chromium, tantalum or molybdenum.
However, as the active matrix LCD having an independent wired storage capacitor described above has capacitor electrodes of opaque metal within each pixel, the aperture ratio decreases.
Moreover, as current LCD must be designed such that spaces between pixel electrode and display signal line, between the display signal line and an adjacent display signal line thereto, and the scanning signal line and an adjacent scanning signal line thereto are minimized so as to improve the aperture ratio thereof, there occurs a parasitic capacitance between pixel electrode and the display signal lines and between pixel electrode and the scanning signal lines due to a voltage variation of signals applied to the scanning signal line and the display signal line and thereby there occur illuminance spots and crosstalks.
In addition, in order to realize the coloration of the active matrix LCD, at the step of combining a lower substrate having the structure as described above with an upper substrate having color filter layers, it is very difficult to accurately align the color filter layers with pixel electrodes, respectively. Accordingly, when each of the pixel electrodes is not precisely aligned with the corresponding one of the color filter layers, the light leakage occurs. Furthermore, if black matrices for separating the color filters on the upper substrate from each other are widened to prevent such light leakage, the aperture ratio of the LCD is lowered.